1. Field of the Invention
The present invention relates to a method for thermally treating lactoferrin without impairing its physiological function, the aforesaid lactoferrin being a pharmacodynamically important milk protein.
2. Description of the Prior Art
Lactoferrin is an iron-binding glycoprotein present in an external secretion of milk and the like, and it has various physiological activities such as bacteriostatic action to pathogenic germs, differentiation regulatory action for leukopenia, enhancement of microbicidal activity of neutrophils, proliferation action of lymphocytes, and iron-absorption regulation action. Therefore, the lactoferrin is the important milk protein not only from a nutritional viewpoint but also from a pharmacodynamical viewpoint.
These physiological functions of lactoferrin arise mainly from the iron-binding ability of the lactoferrin, which exerts its ability to chelate-bind iron only when the three-dimensional stereostructure of the lactoferrin is maintained.
However, when the three-dimensional stereostructure of the lactoferrin is destroyed by heating, the lactoferrin precipitates. In fact, even if the concentration of the lactoferrin is as low as about 0.5%, the lactoferrin precipitates merely by heating at 60.degree. C. for 15 minutes. Therefore, only filtration is employed as a means for removing bacteria from an aqueous lactoferrin solution for the sake of sterilization.
When lactoferrin is utilized for the purpose of enriching iron in food and drink, pasteurizing the lactoferrin is essential. For example, the refreshing beverage manufacturing standards of the Japanese Food Sanitation Act require that (1) when the pH is 4.0 or less, heating at 65.degree. C. for 10 minutes or pasteurization having an effect equal to or greater than this heating shall be performed, and (2) when the pH is 4.0 or more, heating at 85.degree. C. for 30 minutes or pasteurization having an effect equal to or greater than this heating shall be performed. Consequently, lactoferrin which has undergone the filter bacteria removal by filtration cannot be used as an ingredient in refreshing beverages and the like.
Furthermore, when an attempt is made to produce the lactoferrin in powder form by heating and then spray-drying an aqueous lactoferrin solution, precipitates are formed, so that spray nozzles become clogged and the operation cannot be continued. In addition, the iron-binding ability which is the desired property of the lactoferrin is lost by the heating, and the physiological function is also undesirably impaired.
In this connection, although freeze-drying can be employed to prepare lactoferrin, such means increases a manufacturing cost. Thus, this means is not advantageous.